The invention relates to a free-piston engine having a fluid energy unit.
In a known embodiment of such a free-piston engine (see EP-A-0 254 353), the supply and discharge means of the displacement chambers consist of a supply and discharge channel having a non-return valve. At the end of the movement of the piston from the second to the first position, that is at the end of the expansion stroke, the non-return valve of the discharge channel closes so that the pressure in the respective chamber becomes lower than the pressure in the accumulator together with the spring pressure of the valve. The closing action of the non-return valve should take place quickly because otherwise hydraulic liquid will flow back to the chamber which will cause the piston to move back to the second position along some distance, while it is the intention to retain the piston in its first position until a new compression and expansion stroke is required. The quick closing action of the non-return valve necessitates a high spring pressure in the non-return valve. This results, however, in a high flow resistance when hydraulic liquid flows through this non-return valve during the expansion stroke of the piston causing substantial flow losses.
The object of the present invention is to provide a free-piston engine having a fluid energy unit in which this disadvantage is removed in an effective way.